Touch-sensitive panels are increasingly being used for providing input to computers, electronic measurements and test equipment, gaming devices etc. The panel may be provided with a graphical user interface (GUI) for a user to interact with using e.g. a pointer, stylus or one or more fingers.
To be able to provide touch sensitivity to the panel, cameras may be used to capture light scattered off the point(s) of touch on the panel, or by incorporation of resistive wire grids, capacitive sensors, strain gauges etc. into the panel. The panel may provide interaction capabilities for numerous simultaneous touches.
WO2006/095320 discloses a system that emits and detects light above a panel surface. When a user touches the surface, optical transmissions between in- and outgoing coupling points are blocked and the system detects the presence or absence of light between these points. An energy saving function is disclosed to be run when the system is not in use. If the display screen is contaminated by e.g. fingerprints, the optical transmission path may become unintentionally interrupted and the information retrieved from the system erroneous or incomplete as the contaminated surface becomes insensitive to touches. There is also no guarantee that the whole surface can be covered by optical transmission, as the above surface enablement may require a bezel around the panel. If contaminants are collected in front of one of the emitters or detectors there will always be blocked light paths. Systems that has a bezel around the panel surface are very prone to collecting dust, grease, etc at the bezel position. Placing emitters and detectors at the bezel position is therefore not desired, and it may thus be hard to illuminate the whole surface.
Another touch screen with illumination above a touch surface is disclosed in US2008/0062150. The touch screen implements an energy saving function which reduces the scan speed, i.e. frequency of sequentially activating light sources to emit light across the touch surface, if the touch screen has not been touched for a predetermined time period.
WO2009/077962 discloses a touch screen in the form of a panel using a tomograph including signal flow ports. The tomograph processes signals introduced in the panel and detects changes in the signals caused by touches. The touch sensitive technique can be based on frustrated total internal reflection (FTIR), which means that light is injected into the panel from its borders and propagates through the panel. The algorithms used are not specially adapted to detect changes in an FTIR system. For example, the disclosed tomograph uses detection of transmitted light, but determines the locations of touches based on signals obtained by subtracting a background signal from the detected signals. Nevertheless, an FTIR system has the potential of being less sensitive to contaminants such as fingerprints and grease on the panel, as the contaminants only absorb a fraction of the light along a path through the panel. An FTIR system may thus enable a reliable and robust interaction with users. The inventors have now realized that there is a need for an energy saving function that is specially adapted to the FTIR technology.
Thus, the objective of the present invention is thus to reduce the energy consumption of a touch-sensitive system based on FTIR technology, and in particular reduce the energy consumption when the touch-sensitive system is in use.